Since lithium batteries (typically, lithium ion secondary batteries) that are charged and discharged by the movement of lithium ions between a positive electrode and a negative electrode have a small weight and a high output, the demand for such batteries as power supplies to be carried on vehicles and installed in personal computers and portable terminals is expected to grow significantly in the future.
A typical configuration of a secondary battery of this kind includes an electrode with a configuration in which a material (electrode active material) that can reversibly store and release lithium ions, which are charge carriers, is supported on a conductive material (electrode collector), and the above-mentioned electrode active materials have been studied to realize a further increase in energy density and output. For example, materials having a crystal structure of a layered rock salt type, such as of a NiCo system and a NiCoMn system, or materials having a spinel-type crystal structure such as LiMn2O4 are generally known as positive electrode active materials for lithium secondary batteries that are used for the positive electrode. In addition, lithium-containing metal phosphates with an olivine-type crystal structure (referred to hereinbelow simply as “olivine-type phosphates”) represented by the general formula LiMPO4 (M is an element of at least one kind from among Co, Ni, Mn, and Fe) have also been suggested as positive electrode active materials with a high theoretical capacity, low cost, and excellent stability.
The above-mentioned olivine-type phosphates are generally synthesized by a solid-phase synthesis method by which powdered compounds serving as starting materials are weighed to obtain a predetermined composition, mixed, and heat treated (calcined), and a liquid-phase method by which the starting materials are mixed homogeneously in a liquid phase and then calcined. An attempt has been made to increase electric conductivity of the olivine-type phosphates as positive electrode active materials by adding a conductive material (for example, a carbon powder) to the positive electrode active material in the synthesis process or thereafter. For example, Patent Document 1 discloses a method for manufacturing an olivine-type phosphate compound in which conductive paths made of carbon are incorporated into the particles by mixing a starting material mixture of the positive electrode material and an organic material and then calcining. Further, Patent Document 2 discloses a method for manufacturing a positive electrode material for a lithium ion battery in which olivine-type phosphate compounds are bonded together by carbon generated by thermal decomposition of reduced sugar.
Patent Document 1: Japanese Patent Application Laid-open No. 2003-203628
Patent Document 2: Japanese Patent Application Laid-open No. 2007-250417